A known electric bicycle (power-assisted bicycle) includes a power storage such as a battery and an electric motor powered by the power storage. Such an electric bicycle can be easily driven even on an uphill road by adding an auxiliary driving force (assist force) of the electric motor to a pedal force (human driving force) from a pedal. In such an electric bicycle, a force corresponding to a pedal force applied to the pedal is detected by a torque sensor, and then an auxiliary driving force corresponding to the pedal force is generated from the electric motor.
In this case, as a method of detecting a force corresponding to a pedal force by means of the torque sensor, the torque sensor is disposed near, for example, a crankshaft to detect a force transmitted from the crankshaft to a chain ring (also called a front sprocket or a crank gear), or the torque sensor is disposed in, for example, the hub of a rear wheel to detect a force transmitted to the rear wheel through a chain.
In the method of detecting a force transmitted to the rear wheel through the chain, however, the torque sensor detects the force through a force transmission path such as the chain. This may change or lose the direction of a force or an amount of a force through the force transmission path such as the chain, preventing a direct measurement of a force to the pedal so as to reduce correspondence with a pedal force.
In contrast, in the method of detecting a force transmitted from the crankshaft to the chain ring, the torque sensor does not detect the force through the transmission path such as a chain. Thus, a pedal force can be measured in a relatively satisfactory manner.
If a force transmitted from the crankshaft to the crank gear is detected by the torque sensor, conventionally, a motor unit containing the electric motor via a bracket is disposed below, for example, a hanger (also called a bottom bracket) that connects a lower pipe (also called a main pipe), a vertical pipe (also called a seat tube), and a chain stay. The torque sensor is disposed at the front of the motor unit. Specifically, the crankshaft is rotationally supported at the front of the motor unit disposed below the hanger, a torque detecting cylinder that transmits the rotation of the crankshaft to the crank gear is disposed around the crankshaft coaxially with the crankshaft, and the torque sensor is attached to the torque detecting cylinder (Patent Literature 1, etc.).
In a typical bicycle, however, a crankshaft is rotatably supported at the location of the hanger. Thus, a structure that supports a crank with the motor unit disposed below the hanger is a frame structure that is different from that of the typical bicycle, disadvantageously leading to uncomfortable feeling and higher manufacturing cost. Moreover, in order to set the crankshaft as high as that of the typical bicycle from the ground, the hanger connected to the lower pipe and so on needs to be disposed in the upper part of the bicycle, leading to the need for locating the lower pipe as well in the upper part of the bicycle. This disadvantageously makes it difficult for a rider to move a foot over the lower pipe when the rider rides on or gets off the electric bicycle.
In order to overcome the drawbacks, in electric bicycles disclosed in Patent Literatures 2 and 3 and so on, a crankshaft is rotatably supported at the location of a hanger as in the typical bicycle, the hanger containing a torque sensor. With this configuration, the frame structure of the electric bicycle is similar to that of the typical bicycle, thereby suppressing uncomfortable feeling and an increase in manufacturing cost to a certain level. Moreover, the hanger and the lower pipe are located as in the typical bicycle, allowing a rider to easily move a foot over the lower pipe.